The invention relates to a process for producing thin absorbent planar structures which may be used in hygiene products such as diapers, incontinence products, wound contact materials or sanitary napkins to absorb body fluids.
Hygiene products or hygiene articles is a term used principally to designate products such as diapers, sanitary napkins, incontinence products or wound contact materials. Depending on their intended use, these hygiene products are suitable for absorbing and storing body fluids such as blood, urine or wound liquid. Whereas, formerly, the active absorbent material contained in the hygiene products consisted predominantly of pure cellulose, modern hygiene products are usually multilayer constructions, with the individual layers assigned specific functions. A distinction is made between a) layers which face the body, b) layers which face the clothing or which lie on the side of the respective hygiene product which is opposite the aforementioned layers, and c) layers located within the interior of the hygiene product.
Layers which are located on the body-facing side of the hygiene product have functions including, firstly, to ensure transport of the body fluids into the interior of said product. Secondly, they are also intended to prevent rewetting; i.e., the body fluids in the interior of the hygiene product in question should not pass back onto the skin. Layers which are located on the clothing-facing side of the hygiene product, in contrast, constitute an impenetrable barrier for the body fluids in both directions. Their principal function, consequently, is to prevent the soiling of clothes or other materials that are in contact with the respective hygiene product.
The most important component of any hygiene product are absorbent planar structures, which are (generally) located in the interior of the hygiene product in question and are also referred to as absorption layers or inner layers. The absorbent planar structures serve to absorb and store the body liquids and include as their principal components a backing material and a swellable substance. The backing material, which serves to distribute the body fluids, is generally constructed of two or more (backing) layers which are bonded adhesively to one another. The swellable substance, which is used to absorb and/or store the body fluids, is incorporated into the backing material and is also termed the absorbent or superabsorbent.
Besides the swelling capacity specific to each individual substance, the fluid absorption capacity of any hygiene product also depends on the amount of absorbent it incorporates. However, it must be borne in mind that only a limited amount of absorbent can be incorporated into the respective hygiene products. The criteria here include not only the stability of the product as a whole but also, in the case of diapers or sanitary napkins in particular, the decrease in wear comfort that accompanies increases in product thickness. Furthermore, especially for relatively large numbers of product units, it is the case that the thinner the product, the easier it is to store and to transport. It should be taken into account here that it is primarily the inner layers, namely those which comprise the absorbent, that contribute to the overall thickness of the hygiene product. Accordingly, the aim is to produce very thin—and hence also lighter—hygiene products and absorbent planar structures which nevertheless possess a high fluid absorption capacity.
The main problem associated with the production of thin absorbent planar structures and, consequently, of hygiene products which are thin overall is the stable fixing of as large as possible an amount of absorbent on or to the backing material. The absorbent is fixed to a first backing layer by means, for example, of an adhesive. The absorbent fixed on the first backing layer is covered in turn by a second, normally identical backing layer. This produces a sandwichlike planar structure in which there is a layer of absorbent between two backing layers. If too much adhesive is used when fixing the absorbent to or between the backing layers, the absorbent may be stably fixed but cannot be packed either very densely or uniformly on the backing. This inevitably produces relatively thick absorbent planar structures; moreover, adhesives tend to be expensive. If, on the other hand, too little adhesive is used, the absorbent is not fixed stably on the backing and the individual backing layers are inadequately bonded to one another as well. As a consequence of this, (some) absorbent crumbles out of the absorbent planar structure which, owing to its deficient absorbent charge, is unusable or at least considerably restricted in its absorption capacity. For the quality of an absorbent planar structure, it is also important how and in what sequence the individual components are connected to one another.
EP-A 0 033 235 describes a process for producing products for absorbing body fluids. There, swellable polymer particles are coated with an adhesive before being fixed on the surface of a water-absorbing backing. The coating of the swellable polymer particles with the adhesive may be effected in a variety of ways. For example, the polymer particles may be carefully mixed with the aqueous solution of an adhesive. A disadvantage here is that the resulting mixture must then be dried thoroughly in order to remove the water introduced with the adhesive from the swellable polymer particles. Alternatively, the swellable polymer particles may be in the form of a slurry in an organic solvent, to which the adhesive is added in solid or dissolved form. This likewise necessitates thorough drying of the resultant mixture. The same applies if the adhesive solution form is sprayed onto the surface of the swellable polymer particles. Alternatively, the adhesive may be incorporated into the swellable polymer particles during their preparation process. This latter process, however, involves a very great deal of synthesis complexity.
All of the coating methods described above, however, are hampered by a further disadvantage, since the mixture of swellable polymer and adhesive must be additionally ground before being applied to the backing material, in order to obtain small particles in powder form which can be distributed on the backing material. In this way it is possible to produce absorbent planar structures containing 100 g/m2 swellable polymer based on the surface area of the backing material. The absorbent planar structures are pressed at from 160 to 180° C.
GB-A 2 004 201 describes a further means of fixing absorbent on a backing layer. In this case the absorbent is spread out extensively on the backing and then sprayed with an aqueous solution of an adhesive. A disadvantage of this method is that, through the spray application of the aqueous adhesive solution, the backing layer becomes damp. For this reason, the backing layer must have a considerable minimum thickness, since damp backing layers, especially those based on cellulose, tear very easily and so cannot be processed further.
Absorbent hygiene products comprising at least one layer of a textile material and a swellable natural substance enclosed within it are disclosed in DE-A 43 43 947. Swellable polysaccharides are attached to the backing by means of a divalent crosslinking agent, preferably glyoxal, with the formation of covalent bonds.
Alternatively, formaldehyde-free acrylic adhesives are sprayed onto the backing. In the dry state, however, the hygiene products obtained in this procedure have a considerable total thickness of from to 3 to 7 mm.